1. Field of the Invention
The present invention relates to a method and an apparatus for preparing fatty acid esters by reacting fats and oils with a monohydric alcohol.
2. Prior Art
Esters of fatty acids with monohydric alcohols (hereinafter sometimes referred to as xe2x80x9cfatty acid estersxe2x80x9d) are used as industrial raw materials, raw materials in the production of medicaments, fuels, etc. In the field of fuels, they are particularly used as diesel fuels. Furthermore, the fatty acid esters are used as substitute lubricants for conventional mineral oils.
The fatty acid esters are generally prepared by trans-esterification of fats and oils comprising mainly esters of fatty acids and glycerol, which are called fatty acid triglyceride, with monohydric alcohols.
As a preparation method of fatty acid esters, for example, JP-A 2000-143586 discloses a method comprising reacting methanol with waste soybean oil at 300xc2x0 C., which is higher than the critical temperature of methanol, under a pressure of 6.5 MPa to obtain fatty acid esters. However, this JP-A publication does not describe the treatment of intermediate products such as diglyceride and unreacted reactants after the reaction.
JP-A 2000-109883 discloses a method for preparing fatty acid esters comprising continuously supplying methanol and a mixture of rapeseed oil and soybean oil in a reactor and reacting them at 270xc2x0 C., which is higher than the critical temperature of methanol, under a pressure of 12 MPa. However, the yield of the methyl esters of fatty acids is only 60%, and this JP-A publication does not describe the treatment of intermediate products such as diglyceride and unreacted reactants after the reaction either.
When unreacted reactants remain and/or intermediate products form in the conventional methods for preparing fatty acid esters by reacting fats and oils with monohydric alcohols under conditions where the monohydric alcohols are in the supercritical state, the unreacted reactants and/or the intermediate products should be separated from the resulting fatty acid esters and discharged.
One object of the present invention is to provide a method for preparing a fatty acid ester comprising reacting fats and oils with a monohydric alcohol under conditions where the monohydric alcohol is in a supercritical state, in which the discharge of unreacted reactants and/or intermediate products are suppressed so that the fatty acid ester is obtained in a high yield.
Another object of the present invention is to provide an apparatus suitable for carrying out such a method for preparing a fatty acid ester.
These and other objects of the present invention are achieved by a method for preparing a fatty acid ester comprising reacting fats and oils with a monohydric alcohol in a reactor under conditions where the monohydric alcohol is in a supercritical state, wherein a reaction mixture containing unreacted reactants and/or intermediate products is recycled to the reactor, and an apparatus for preparing a fatty acid ester comprising reacting fats and oils with a monohydric alcohol in a reactor under conditions where the monohydric alcohol is in a supercritical state, wherein the apparatus has a mechanism for recycling a reaction mixture containing unreacted reactants and/or intermediate products to a reactor.